Ex-Liberian President Charles Taylor, jailed for war crimes, is reported to have phoned political allies from inside a UK maximum security prison.
A recording of the alleged call, in which he reportedly advises his party members on tactics, was obtained by Africa Confidential magazine.
He is serving a 50-year sentence for supporting rebels who committed atrocities in Sierra Leone.
The Ministry of Justice said it does not comment on individual cases.
Alain Werner, director of Civitas Maxima, an NGO which builds legal cases against alleged Liberian war criminals, said the allegation was "extremely worrying".
The call, which has been heard by the BBC, is thought to have been made from a landline inside HMP Frankland, near Durham, on 28 January, Taylor’s 69th birthday.
In the phone call:
Taylor’s supporters put him on their phone’s loudspeaker and applause and cheers can be heard as he speaks
He rails against former allies who turned against him and assures supporters that he has never abandoned them
He advises them to avoid people who would betray his party, saying: "Go back to the base and everything will be fine"
He adds: "I’m in a cell but I’m not in prison because my mind cannot be in prison," and describes himself as a "sacrificial lamb"
Africa Confidential also reported that Taylor had been threatening politicians he opposes in other phone calls.
The Residual Special Court for Sierra Leone, the United Nations body which held Taylor’s trial, said it was urgently following up with the British government.
Mr Werner, of NGO Civitas Maxima, said: "That he may interfere from his cell in a British jail on the political life in Liberia is extremely worrying, especially considering that there will be presidential elections later this year."
In Liberia’s election in October, Taylor’s National Patriotic Party (NPP) is in a coalition with the Congress for Democratic Change, which is seeking to elect former football star George Weah, with Taylor’s ex-wife Jewel standing as vice-president.
Why is Taylor in prison in the UK?
Taylor was convicted in 2012 of 11 counts of war crimes, slavery, rape, recruiting child soldiers and aiding and abetting the Revolutionary United Front (RUF) rebels in Sierra Leone during the 1991-2002 civil war, in which some 50,000 people died.
The former Liberian leader was found to have supplied weapons to the RUF in exchange for so-called blood diamonds.
The rebels were notorious for hacking off the limbs of civilians to terrorise the population.
The UN, which convened a Special Court on Sierra Leone, issued an appeal for a country to host him after his conviction and the UK volunteered.
An act of parliament was passed to allow for Taylor to serve his sentence in the UK, at the cost of the British government.
His trial was held in the Netherlands in case it sparked renewed unrest in West Africa.
Prison guidelines state prisoners should not communicate any material which would create a threat or risk of violence or physical harm to any person, or endanger public safety.
- Former Liberian leader Charles Taylor